Hitherto, polyurethane resins have been used in a wide variety of forms including foams, adhesives, fibers, elastomers, and paints.
They are prepared mainly by a reaction of an organic diisocyanate compound with polyol compounds.
The polyol compounds which have been employed include polyether polyols such as polypropylene glycols, polytetramethylene glycols, polyester polyols which are derived from divalent carboxylic acids such as adipic acid, and polyhydric alcohols, and polylactone polyols which are obtained by a reaction of lactones with alcohols.
A variety of polyol compounds have been used to produce polyurethane resin which is used for many kinds of purposes.
However, as polyether polyols have ether bonds, the urethane resin which is produced by employing them has the disadvantage of being poor in heat resistibility and in weatherability.
The urethane resin which is produced by employing polyester of polylactone polyols has the disadvantage of being poor in water resistibility, thought to be due to presence of ester bonds.
The use of polyols having carbonate bonds in their molecular structure has been proposed to produce novel urethane resins which overcome the foregoing disadvantages.
The polycarbonatepolyol which is used more widely than any other type of polyol compound is the polyol having a carbonate bond in the molecular structure containing 1,6-hexanediol in the main chain of the molecular structure, as shown by formula (I): ##STR1##
The polycarbonatediol having 1,6-hexanediol structures in the main chain can produce a polyurethane resin having a very good balance of various properties, including mechanical strength, excellent resistibility under high heat and moisture conditions, etc., thereby maintaining a good balance of properties, and has also the advantage of being easy to produce on an industrial basis.
The polycarbonatediol having 1,6-hexanediol structures the main chain, however, has a melting temperature range of 40.degree. to 50.degree. C., therefore, it is a relatively hard wax-like solid at ordinary temperatures.
Accordingly, it is required to be melted before use as a raw material for manufacturing urethane resins, etc., using a tank for heating and melting.
Furthermore, the use of the tank for heating and melting is inevitably required to prevent heat radiation from the surface of the tank and piping. Accordingly, it is disadvantageous not only from the viewpoint of energy costs, but also plant costs.
For the purpose of overcoming the above disadvantages, a random copolymer composed of .epsilon.-caprolactone and dialkylcarbonate such as diethylcarbonate or dimethylcarbonate had been disclosed in Japanese Publication Laid-open No. 115925/1985 (entitled: A Process for Producing Polyurethane).
The random copolymer is liquid polyol having a low melting temperature.
However a polyurethane prepared with said randomly copolymerized polyol has a disadvantage of being poor in heat and moisture resistibility. Such is thought to be due to ester bonds based on .epsilon.-caprolactone in the molecules. The inventors of this invention have now found that it is possible to solve the problems as hereinabove pointed out and produce polyurethanes having high mechanical strength and at and moisture resistibility, and furthermore having improved properties at low temperatures by employing a particular polycarbonatediol composition which has a low melting temperature and excellent processability.